1. Gum arabic is a water-soluble polysaccharide resistant to human gut enzymes and thus can be described as dietary fibre.2. Using a most-probable-number technique, estimates were made of total anaerobes and of gum-arabic fermenters in the faeces of a volunteer during a contro1 period and during addition of 10 g gum arabic/d to the diet. Using an enrichment technique, the principal bacteria able to utilize gum arabic as the only carbohydrate source were isolated and characterized. 3. Faecal samples were analysed for undegraded gum arabic and, following acid-hydrolysis, for total sugars. 4. The proportion of the faecal flora able to degrade the gum arabic polymer rose from an initial level of 6.5% to more than 50% during gum-arabic ingestion, and subsequently returned to the control level after ingestion ceased. The principal gum-arabic fermenters were species of Bucteroides and Bifdobucterium.5 . Undegraded gum arabic was not detected in any faecal sample nor were there significant differences in the level of total sugars in acid-hydrolysed faeces between gum arabic and control periods.6. The results presented indicate a direct and rapid change in faecal flora in response to a specific change in the diet of a human volunteer.Little change in human faecal flora has been detected following changes in diet (Moore et al. 198 1 ; Savage, 1982) and studies have been complicated by the differences in flora seen between individuals. It is possible, however, that although no gross changes in the species occur, the population may adapt to the presence of new substrates by a change in the biotypes present or in other ways.Dietary fibre, which has a marked effect on the function of the large intestine, increasing faecal weight and reducing transit time (Eastwood & Passmore, 1983), includes the plant gums widely used as thickening, gelling and emulsifying agents in the food industry. Previous work has shown that inclusion of guar gum in the diet of a human subject increased the total count of anaerobic bacteria in faeces (Bayliss & Houston, 1985).Gum arabic is the second most widely used gum in the food industry (Blenford, 1984) and as such forms part of the average Western diet, although at a low level. It is a highly water-soluble extract from Acacia senegal, and is a high-molecular-weight globular molecule with a backbone of D-galactopyranose units and complex side chains of D-galactose, L-arabinose, L-rhamnose and D-glucuronic acid. The in vivo effects of gum arabic in the diet have been demonstrated by McLean Ross et al. (1983), who showed that breath hydrogen increased in subjects who had eaten a diet containing gum arabic for 3 weeks but not in control subjects whose diet had not contained gum arabic. It seems likely that this increase reflected an adaptation of the microbial population of the human colon to the presence of the substrate. Therefore, the present study was carried out to determine whether a specific change in the diet of a human volunteer could lead to a change in faecal flora or in that part of the flo...
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